Peaking amplifier frequency tuning

ABSTRACT

A circuit including: input and output nodes and first and second feedback nodes; a first input amplifier having an input connected to the input node and an output connected to the first feedback node; a second input amplifier having an input connected to the input node and an output connected to the second feedback node; a capacitor connecting the first feedback node and the second feedback node; an amplifier having an input connected to the first feedback node and an output connected to the output node; a base feedback amplifier with an input connected to the output node and an output connected to the first feedback node; a tunable feedback amplifier with an input connected to the output node and an output connected to the second feedback node; and a tuning circuit for varying a transconductance of the tunable feedback circuit and operational frequency of the peaking amplifier circuit.

TECHNICAL FIELD

The subject matter disclosed herein relates generally to transmissionsystems, and more particularly to high speed peaking amplifier frequencytuning.

BACKGROUND

With the ever present demand for higher data rates in serial links,transmission frequencies have continued to increase. Unfortunately, thevast majority of communication channels suffer high losses at higherfrequencies. To improve the maximum data rates of such links, it isoften necessary to equalize the frequency response of the channel sothat pulse distortion is reduced. For this reason, the receivers ofmodern high-speed data communication links commonly employ peakingamplifiers, which boost the high-frequency components of the receivedsignal that were attenuated by the channel response.

Many peaking amplifiers use some combination of series inductive and/orshunt capacitive loading to generate a high gain at a certain frequencywhile suppressing the signal at other frequencies. While an effectivesolution, this type of peaking amplifier possesses an inherent tradeoffbetween bandwidth and loss/noise, with many peaking amplifiers focusingon a narrowband response with high signal integrity. While most of thesepeaking amplifiers are only meant to operate at a specific frequency,this narrowband response becomes problematic over PVT (process, voltage,temperature) variation, where the response of the peaking amplifier mayshift such that the operating frequency is out of band.

SUMMARY

The subject matter disclosed herein relates generally to transmissionsystems, and more particularly to high speed peaking amplifier frequencytuning.

A first aspect includes a tunable peaking amplifier circuit, including:an input node, an output node, a first feedback node, and a secondfeedback node; a first input amplifier having an input connected to theinput node and an output connected to the first feedback node; a secondinput amplifier having an input connected to the input node and anoutput connected to the second feedback node; a coupling capacitorconnected between the first feedback node and the second feedback node;an amplifier having an input connected to the first feedback node and anoutput connected to the output node; a base feedback amplifier in anegative feedback loop with an input connected to the output node and anoutput connected to the first feedback node; a tunable feedbackamplifier in a negative feedback loop with an input connected to theoutput node and an output connected to the second feedback node; and atuning circuit for varying a transconductance of the tunable feedbackcircuit to adjust an operational frequency of the peaking amplifiercircuit

A second aspect includes a peaking amplifier circuit, including: aninput node, an output node, a first feedback node, and a second feedbacknode; a first input amplifier having an input connected to the inputnode and an output connected to the first feedback node; a second inputamplifier having an input connected to the input node and an outputconnected to the second feedback node; a coupling capacitor connectedbetween the first feedback node and the second feedback node; anamplifier having an input connected to the first feedback node and anoutput connected to the output node; a base feedback amplifier in anegative feedback loop with an input connected to the output node and anoutput connected to the first feedback node; a capacitor and a loadimpedance in series connected from the first feedback node to a sourcevoltage. a first tunable feedback amplifier in a negative feedback loopwith an input connected to the output node and an output connected tothe second feedback node; a second tunable feedback amplifier in apositive feedback loop with an input connected to the output node and anoutput connected to the second feedback node; and a tuning circuit forapplying a first tuning voltage to the first tunable feedback amplifierand for applying a second tuning voltage to the second tunable feedbackamplifier.

A third aspect includes a peaking amplifier circuit, including: a firstinput amplifier having an input connected to an input node and an outputconnected to a first feedback node; a second input amplifier having aninput connected to the input node and an output connected to a secondfeedback node; a coupling capacitor connected between the first feedbacknode and the second feedback node; an amplifier with an input connectedto the first feedback node and an output connected to an output node; anuntuned feedback amplifier with an input connected to the output nodeand an output connected to the first feedback node; a first tunablefeedback amplifier with an input connected to the output node and anoutput connected to the second feedback node; a second tunable feedbackamplifier with an input connected to the output node and an outputconnected to the second feedback node; and a tuning circuit forselectively applying a first tuning voltage to the first tunablefeedback amplifier and a second tuning voltage to the second tunablefeedback amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this disclosure will be more readilyunderstood from the following detailed description of the variousaspects of the disclosure taken in conjunction with the accompanyingdrawings that depict various embodiments of the disclosure.

FIG. 1 depicts a frequency tunable peaking amplifier according toembodiments.

FIG. 2 depicts the frequency tunable peaking amplifier of FIG. 1 dividedinto two stages according to embodiments.

FIG. 3 depicts a frequency tunable peaking amplifier with fine tuningaccording to embodiments.

FIG. 4 depicts a chart of the frequency response of an illustrativepeaking amplifier with fine tuning according to embodiments.

FIG. 5 depicts a frequency tunable peaking amplifier with coarse tuningaccording to embodiments.

FIG. 6 depicts the selective activation/deactivation of one or morefeedback amplifiers using input switches.

FIG. 7 depicts a frequency tunable peaking amplifier with coarse tuningaccording to embodiments.

FIG. 8 depicts a frequency tunable peaking amplifier with fine andcoarse tuning according to embodiments.

FIG. 9 depicts a chart of the frequency response of an illustrativepeaking amplifier with fine and coarse tuning according to embodiments.

FIG. 10 depicts a frequency tunable peaking amplifier with reduced DCgain attenuation variation according to embodiments.

FIG. 11 depicts a chart of the frequency response of the peakingamplifier of FIG. 10 according to embodiments.

FIG. 12 depicts a frequency tunable peaking amplifier with reduced DCgain attenuation variation according to embodiments.

FIG. 13 depicts a chart of the frequency response of the peakingamplifier of FIG. 12 according to embodiments.

FIG. 14 depicts a frequency tunable peaking amplifier with reduced DCgain attenuation variation, gain equalization, and low end extensionaccording to embodiments.

FIG. 15 depicts a chart of the frequency response of the peakingamplifier of FIG. 14 according to embodiments.

FIG. 16 depicts a chart of the frequency response of the peakingamplifier of FIG. 15 according to embodiments.

It is noted that the drawings of the disclosure are not necessarily toscale. The drawings are intended to depict only typical aspects of thedisclosure, and therefore should not be considered as limiting the scopeof the disclosure. In the drawings, like numbering represents likeelements between the drawings.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanyingdrawings that form a part thereof, and in which is shown by way ofillustration specific exemplary embodiments in which the presentteachings may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice thepresent teachings, and it is to be understood that other embodiments maybe used and that changes may be made without departing from the scope ofthe present teachings. The following description is, therefore, merelyillustrative. It is understood that the various process steps discussedherein can be implemented in the same manner and/or with slightmodifications.

Referring to FIG. 1, there is shown a frequency tunable peakingamplifier 10 according to embodiments. The peaking amplifier 10 may, forexample, be a Serializer/Deserializer (SERDES) type peaking amplifier.In general, the peaking amplifier 10 includes an input amplifier A, aninput amplifier B, an amplifier D, and a feedback amplifier FB. Theinput amplifiers A and B have inputs that are commonly connected to aninput voltage V_(In). The output of the input amplifier A is connectedto a feedback node 12. The output of the input amplifier B iscapacitively coupled to the output of the amplifier A by a couplingcapacitor C. The output of the input amplifier A is connected to a loadresistor R_(A). The load resistor R_(A) is connected between thefeedback node 12 and a supply voltage.

The input of the amplifier D is connected to the feedback node 12, andthe output of amplifier D is connected to an output node V_(Out) of thepeaking amplifier 10. The input of the feedback amplifier FB isconnected to the output node V_(Out) of the peaking amplifier 10, andthe output of the feedback amplifier FB is connected to the feedbacknode 12. The feedback amplifier FB shares the load resistor R_(A) withinput amplifier A.

The basic function of the peaking amplifier 10 is outlined as follows:The input amplifier A provides a relatively low transconductance g_(mA)from V_(In) to V₁ (at the feedback node 12) at all frequencies, whilethe amplifier D provides a much higher transconductance (e.g., >10g_(mA)) from V₁ to V_(Out). The feedback amplifier FB attempts to bringdown the magnitude of V_(Out) through a negative feedback loop. Thefeedback transconductance g_(mFB) of the feedback amplifier FB isapproximately equal to −g_(mA).

The amplifier B provides a large transconductance g_(mB) (e.g., also >10g_(mA)), which is AC coupled into the feedback node 12 through thecoupling capacitor C. At low frequencies, g_(mB) is isolated from theoutput. However, as the input frequency increases, the impedance of thecoupling capacitor C drops, allowing g_(mB) to overpower the negativefeedback of the feedback amplifier FB, increasing the overall gain ofthe peaking amplifier 10. The peaking frequency of the peaking amplifier10 may be controlled, for example, by adjusting the inductive componentsof load impedances Z_(B) and Z_(D).

The overall transfer function of the peaking amplifier 10 is given bythe following equation:

$\frac{V_{out}}{V_{in}} = {\frac{G_{A}G_{2}}{1 + {G_{FB}G_{2}}} \cdot \frac{1 + {{sCZ}_{B}( {1 + \frac{{\mathcal{g}}_{mB}}{{\mathcal{g}}_{mA}}} )}}{1 + {{sCZ}_{B}( {1 + \frac{R_{A}\text{/}Z_{B}}{1 + {G_{FB}G_{2}}}} )}}}$Where:G _(A) =g _(mA) ·R _(A)G _(B) =g _(mB) ·Z _(B)G _(D) =g _(mD) ·Z _(D)G _(FB) =g _(mFB) ·R _(A)

To better understand the peaking amplifier frequency tuning disclosedherein, let us consider the peaking amplifier 10 as a cascaded system asshown in FIG. 2 with a first stage S1 including the amplifiers A and B,and a second, feedback stage S2 containing the feedback loop. To thisextent, the transfer function of the feedback stage S2 may be describedby the following equation:

$\frac{V_{out}}{V_{1}} = \frac{\alpha}{1 + {\alpha\beta}}$ whereα = G_(D) and$\beta = {{\mathcal{g}}_{mFB} \cdot \frac{R_{A}( {1 + {sCZ}_{B}} )}{1 + {{sC}( {R_{A} + Z_{B}} )}}}$Then, it follows that:

$\frac{V_{out}}{V_{1}} = \frac{\alpha( {1 + {{sC}( {Z_{B} + R_{A}} )}} }{1 + {{\alpha\mathcal{g}}_{mFB}R_{A}} + {{sC}( {Z_{B} + R_{A} + {{sC}( {Z_{B} + R_{A} + {{\mathcal{g}}_{mFB}R_{A}Z_{B}}} )}} }}$From this transfer function, the frequency of the dominant pole can bedetermined to be:

$\omega_{p} = \frac{1}{{CZ}_{B}( {1 + \frac{R_{A}\text{/}Z_{B}}{1 + {G_{FB}G_{D}}}} )}$

It has been observed, in accordance with the above equation, that theoperating frequency can be increased, and thus the frequency rolloffpushed higher, by increasing the G_(FB) term, which is the product ofg_(mFB) and R_(A). Similarly, the operating frequency can be decreased,and thus the frequency rolloff pushed lower, by decreasing the G_(FB)term. While R_(A) may in some cases be difficult to adjust electrically,g_(mFB) can be electrically adjusted, as presented in detail below andgenerally indicated by arrow 14 in FIGS. 1 and 2, by suitably tuning thefeedback stage S2 of the peaking amplifier 10. In addition, g_(mFB) canbe changed with minimal impact on the forward gain path in the peakingamplifier 10, while both amplifiers A and B push their transconductancethrough R_(A). To this extent, according to embodiments, the operatingfrequency of the peaking amplifier 10 can be moved or “tuned” withoutadversely affecting the gain of the peaking amplifier 10 at thatfrequency.

One technique, for tuning the feedback stage S2 of the peaking amplifier10, depicted in FIG. 3, involves adjusting the bias voltage (V_(Tune))of the tail current in the feedback amplifier FB, which will accordinglyvary the transconductance g_(mFB) of the feedback amplifier FB.According to embodiments, there is a monotonic relationship betweenV_(Tune) and g_(mFB), such that g_(mFB) increases as V_(Tune) isincreased, and g_(mFB) decreases as V_(Tune) is decreased. By varyingthe transconductance g_(mFB) of the feedback amplifier FB using tailcurrent tuning (via V_(Tune)), the pole of the feedback path can beadjusted as described above according to:

$\omega_{p} = \frac{1}{{CZ}_{B}( {1 + \frac{R_{A}\text{/}Z_{B}}{1 + {G_{FB}G_{D}}}} )}$Thus,Higher g _(mFB)→Higher ω_(p)→Higher ω_(peak)andLower g _(mFB)→Lower ω_(p)→Lower ω_(peak).

In a particular embodiment, the tail transistor in the feedbackamplifier FB has a threshold voltage V_(t)=˜290 mV. In this case, thepeak frequency of the peaking amplifier 10 can continue to be adjustedwith V_(Tune) below this value, gradually cutting out the feedback path.Above a V_(Tune) of ˜400 mV, the drain voltage cannot keep the inputtransistor in saturation, and performance suffers accordingly. Thus, inthis example, a suitable tuning range for V_(Tune) is set toapproximately 190 mV-390 mV. For practical purposes, this tuning may beaccomplished, for example, using a tunable current mirror. If mirroredthrough an identical device, the input current will vary fromapproximately 10 μA-50 μA.

A chart of the frequency response of an illustrative peaking amplifier10 with fine tuning according to embodiments is presented in FIG. 4. Asshown, the tuning bandwidth of the peaking amplifier 10, when tunedusing V_(Tune) (FIG. 3), is approximately 15.6 GHz-25.2 GHz. Within thisbandwidth, there is a minimal variation in the high frequency gain fromapproximately 12.4 dB-11.9 dB.

The use of V_(Tune) allows for a “fine” adjustment of thetransconductance g_(mFB) of the feedback amplifier FB. This fine tuningmay be performed, for example, by adjusting the bias current (e.g.,through V_(Tune)) of the feedback amplifier FB which, as describedabove, results in a monotonic change in the transconductance g_(mFB). Byvarying the transconductance g_(mFB) of the feedback amplifier FB inthis manner, the pole of the feedback path can be adjusted as describedabove.

According to other embodiments, as depicted in FIG. 5, a “coarse” tuningof the total transconductance g_(ms2) of the feedback amplifiers (e.g.,feedback amplifier FB and feedback amplifier cells FB_(Cell)) in thefeedback stage S2 can be provided by switching one or more feedbackamplifier cells FB_(Cell) into or out of the feedback stage S2 of afrequency tunable peaking amplifier 20. This results in a linearincrease or decrease, respectively, of the total transconductanceg_(ms2) of the feedback amplifiers in the feedback stage S2, and acorresponding increase or decrease, respectively, of the pole of thefeedback stage S2. Each feedback amplifier cell FB_(Cell) may providethe same amount of transconductance g_(mFB), or may provide differentlevels of transconductance g_(mFB).

The switching of a feedback amplifier cell FB_(Cell) into or out of thefeedback stage S2 of the peaking amplifier 20 may be actuated using aswitch 16 at the input of the feedback amplifier cell FB_(Cell) as shownin FIG. 5. Tuning can be provided as shown in FIG. 6 by selectivelyactivating/deactivating one or more of the feedback amplifier cellsFB_(Cell) using respective input switches 16. While tuning in thismanner may be sufficient for some applications, it requires switches 16capable of handling a high frequency signal, and may result in a largecapacitive variation at the output.

To avoid issues related to the use of a switch 16 at the input of afeedback amplifier cell FB_(Cell), switching may be accomplished at thegate of the tail transistor in the feedback amplifier cell FB_(Cell).This is the same node in the feedback amplifier FB at which V_(Tune) isapplied for fine tuning. For example, as shown in FIG. 7, a feedbackamplifier cell FB_(Cell) of a frequency tunable peaking amplifier 30 maybe switched off by applying a V_(Tune)=V_(SS) (e.g., ground) to the gateof the tail transistor in the feedback amplifier cell FB_(Cell). To thisextent, the feedback path provided by a feedback amplifier cellFB_(Cell) can be eliminated simply by applying a voltage V_(Tune) thatcauses the tail transistor in the feedback amplifier cell FB_(Cell) toturn off.

The concepts of fine and coarse tuning in a frequency tunable peakingamplifier 40 can be combined into a single implementation as shown inFIG. 8. In this embodiment, a “fine” adjustment of the transconductanceg_(mFB) of the feedback amplifier FB, and thus of the totaltransconductance g_(ms2) of the feedback amplifiers in the feedbackstage S2, may be provided by adjusting the bias current V_(Tune-Fine)applied to the gate of the tail transistor in the feedback amplifier FB.By varying the transconductance g_(mFB) of the feedback amplifier FB inthis manner, a fine adjustment of the pole of the feedback path can beobtained as described above.

Still referring to FIG. 8, a “coarse” tuning of the totaltransconductance g_(ms2) of the feedback stage S2 can be provided byswitching one or more feedback amplifier cells FB_(Cell) into or out ofthe feedback stage S2 of the peaking amplifier 40. For example, a givenfeedback amplifier cell FB_(Cell) may be switched off by applying aV_(Tune-Coarse)=V_(SS) (e.g., ground) to the gate of the tail transistorin the feedback amplifier cell FB_(Cell). This results in a linearchange of the total transconductance g_(ms2) of the feedback stage S2,and a corresponding change of the pole of the feedback stage S2.

A chart of the frequency response of an illustrative peaking amplifier40 with fine and coarse tuning according to embodiments is presented inFIG. 9. As shown, the tuning bandwidth of the peaking amplifier 40 withfine and coarse tuning is approximately 14.2 GHz-26.7 GHz. Within thisbandwidth, there is a minimal variation in the high frequency gain fromapproximately 17.5 dB-16.6 dB. Comparing FIGS. 4 and 9, it can be seenthat in both cases the tuning mechanism(s) disclosed herein provide apeaking amplifier with a multi-GHz tuning range (instead of a singlefrequency). In addition, the use of fine and coarse tuning may increasethe tuning range of a peaking amplifier compared to just fine tuning.

A control circuit may be used in any of the embodiments of the frequencytunable peaking amplifiers 10, 20, 30, 40, 50 (described below), 60(described below), and 70 (described below) disclosed herein to controlthe transconductance of the feedback stage S2 and adjust the operationalfrequency of the peaking amplifier. Other components of the controlcircuit may be included to provide control schemes for adjusting thevalue of the transconductance of the feedback stage S2. For example, adynamic control scheme may be implemented, wherein the transconductanceof the feedback stage S2 is dynamically adjusted under changingoperating conditions. In particular, a control circuit can be employedto receive as input certain data regarding operating conditions, e.g.,data rates, channel loss, etc., and then dynamically output a controlsignal (e.g., tuning voltage) to dynamically adjust the value of thetransconductance of the feedback stage S2 to adjust the operationalfrequency of the peaking amplifier.

It can be seen from FIGS. 4 and 9 that the widening of the tuningbandwidth provided by the above-described embodiments of the peakingamplifier may be accompanied by a corresponding variation in DC gainattenuation. This relationship can be better understood by referringagain to Equation 1, reproduced below:

$\begin{matrix}{\frac{V_{out}}{V_{in}} = {\frac{G_{A}G_{2}}{1 + {G_{FB}G_{2}}} \cdot \frac{1 + {{sCZ}_{B}( {1 + \frac{{\mathcal{g}}_{mB}}{{\mathcal{g}}_{mA}}} )}}{1 + {{sCZ}_{B}( {1 + \frac{R_{A}\text{/}Z_{B}}{1 + {G_{FB}G_{2}}}} )}}}} & ( {{Equation}\mspace{14mu} 1} )\end{matrix}$As evident from Equation 1, an increase in g_(mFB) results in acorresponding increase in G_(FB) (recall that G_(FB)=g_(mFB)·R_(A)),which results in a decrease in the overall gain of the peakingamplifier. While this is, of course, beneficial to the high frequencyresponse, the variation in the DC gain attenuation may not be suitablefor some applications. To this extent, additional embodiments of apeaking amplifier which counteract such a variation in DC gainattenuation are presented below.

A frequency tunable peaking amplifier 50 for counteracting variations inDC gain attenuation according to embodiments is depicted in FIG. 10.Similar to the peaking amplifier 10 described above with regard to FIG.3, the peaking amplifier 50 includes an input amplifier A, an inputamplifier B, and an amplifier D. The input amplifiers A and B haveinputs that are commonly connected to an input voltage V_(In). Theoutput of the input amplifier A is connected to a feedback node 12. Theoutput of the input amplifier B is capacitively coupled to the output ofthe amplifier A by a coupling capacitor C₁. The output of the inputamplifier A is connected to a load resistor R_(A). The load resistorR_(A) is connected between the feedback node 12 and a supply voltage.

The input of the amplifier D is connected to the feedback node 12, whilethe output of amplifier D is connected to an output node V_(Out) of thepeaking amplifier 50. Load impedances Z_(B) and Z_(D) are present at theoutputs of the input amplifier B and the amplifier D, respectively.

Unlike the peaking amplifier 10, the peaking amplifier 50 includes anuntuned, base feedback amplifier FB_(Base), which is always on, and atunable feedback amplifier FB_(Tune). The tunable feedback amplifierFB_(Tune) may be tuned using any of the tuning techniques detailed above(e.g., through adjustment of V_(Tune) and/or variation in the number offeedback amplifier cells). The inputs of the base feedback amplifierFB_(Base) and the tunable feedback amplifier FB_(Tune) are connected tothe output node V_(Out) of the peaking amplifier 50. The output of thebase feedback amplifier FB_(Base) is connected to the feedback node 12.The output of the tunable feedback amplifier FB_(Tune) is capacitivelycoupled to the feedback node 12 by a feedback coupling capacitor C_(FB)and is further connected to a load impedance Z_(FB).

The inclusion of the untuned, base feedback amplifier FB_(Base), tunablefeedback amplifier FB_(Tune), coupling capacitor C_(FB), and loadimpedance Z_(FB) in the peaking amplifier 50 results in a reduction ofthe variation in DC gain attenuation. A chart of the frequency responseof an illustrative peaking amplifier 50 is presented in FIG. 11.Comparing FIGS. 4 and 11, it can be seen that there is much lessvariation in DC gain attenuation for the peaking amplifier 50 comparedto the peaking amplifier 10.

In the peaking amplifier 50, the coupling capacitor C_(FB) capacitivelycouples the tunable feedback amplifier FB_(Tune) to the feedback node12. In addition, the load impedance Z_(FB) keeps the tunable feedbackamplifier FB_(Tune) properly biased at DC. This configurationcounteracts the DC gain variation and isolates DC shift from thefeedback node 12. The peaking amplifier 50 provides a practicallyconstant feedback factor at DC, due to the incorporation of the basefeedback amplifier FB_(Base), reducing variation in the DC gainattenuation over the V_(Tune) sweep. The DC operation of the forwardamplification path in the peaking amplifier 50 is completely isolated,allowing for an expanded tuning range.

Although the peaking amplifier 50 provides several advantages (e.g.,deceased DC gain variation), such advantages do not come without thecost of additional components (e.g., coupling capacitor C_(FB) and loadimpedance Z_(FB)), as well as a gain rolloff at higher frequencies. Tothis extent, as depicted in FIG. 12, a frequency peaking amplifier 60 isdisclosed that provides advantages similar to those provided by thepeaking amplifier 50, but without the need for a separate couplingcapacitor C_(FB) and load impedance Z_(FB), and with reduced gainrolloff at higher frequencies.

A frequency tunable peaking amplifier 60 for counteracting variations inDC gain attenuation according to embodiments is depicted in FIG. 12.Similar to the peaking amplifier 50 described above with regard to FIG.10, the peaking amplifier 60 shown in FIG. 12 includes an inputamplifier A, an input amplifier B, and an amplifier D. The inputamplifiers A and B have inputs that are commonly connected to an inputvoltage V_(In). The output of the input amplifier A is connected to afeedback node 12. The output of the input amplifier B is capacitivelycoupled to the output of the amplifier A by a coupling capacitor C. Theoutput of the input amplifier A is connected to a load impedance Z_(A).The load impedance Z_(A) is connected between the feedback node 12 and asupply voltage.

The input of the amplifier D is connected to the feedback node 12, andthe output of amplifier D is connected to an output node V_(Out) of thepeaking amplifier 60. Load impedances Z_(B) and Z_(D) are present at theoutputs of the input amplifier B and the amplifier D, respectively.

The peaking amplifier 60 further includes an untuned, base feedbackamplifier FB_(Base), which is always on, and a tunable feedbackamplifier FB_(Tune). The tunable feedback amplifier FB_(Tune) may betuned using any of the tuning techniques detailed above (e.g., throughadjustment of V_(Tune) and/or variation in the number of feedbackamplifier cells). The inputs of the base feedback amplifier FB_(Base)and the tunable feedback amplifier FB_(Tune) are connected to the outputnode V_(Out) of the peaking amplifier 60. The output of the basefeedback amplifier FB_(Base) is connected to the feedback node 12. Theoutput of the tunable feedback amplifier FB_(Tune) is coupled to theoutput of the input amplifier B and is capacitively coupled to theoutput of the amplifier A by the coupling capacitor C.

A chart of the frequency response of an illustrative peaking amplifier60 is presented in FIG. 13. Comparing FIGS. 11 and 13, it can be seenthat similar to the peaking amplifier 50, there is a reduced variationin DC gain attenuation associated with the peaking amplifier 60.However, rather than having gain rolloff at higher frequencies as occurswith the peaking amplifier 50, the there is less gain rolloff at higherfrequencies for the peaking amplifier 60. In fact, the gain increases asthe frequency increases.

Similar to the peaking amplifier 50, the peaking amplifier 60 provides aconstant feedback factor at DC, due to the incorporation of the basefeedback amplifier FB_(Base), reducing variation in the DC gainattenuation over the V_(Tune) sweep. Unlike the peaking amplifier 50,however, fewer additional capacitors or loads are needed in the peakingamplifier 60. There is less peak loss at high frequencies; gainincreases as the frequency increases. The peaking amplifier 60 has aslightly reduced tuning range compared to the peaking amplifier 50.

FIG. 14 depicts a frequency tunable peaking amplifier 70 with reduced DCgain attenuation variation, gain equalization, and low end extensionaccording to embodiments. The peaking amplifier 70 reduces the gainvariations at higher frequencies experienced by one or more of thepeaking amplifiers disclosed above. While such gain variations may beacceptable in various applications of a frequency tunable peakingamplifier, other applications may require a more consistent gain acrossthe tuning bandwidth of a frequency tunable peaking amplifier.

The peaking amplifier 70 includes an input amplifier A, an inputamplifier B, and an amplifier D. The input amplifiers A and B haveinputs that are commonly connected to an input voltage V_(In). Theoutput of the input amplifier A is connected to a feedback node 12. Theoutput of the input amplifier B is connected to a feedback node 22 andis capacitively coupled to the output of the amplifier A by a couplingcapacitor C₁. The output of the input amplifier A is connected to a loadimpedance Z_(A). Further, the output of the input amplifier A isconnected to a series arrangement of a feedback coupling capacitorC_(FB) and a load impedance Z_(FB), which is connected to a sourcevoltage.

The input of the amplifier D is connected to the feedback node 12, whilethe output of amplifier D is connected to an output node V_(Out) of thepeaking amplifier 70. Load impedances Z_(B) and Z_(D) are present at theoutputs of the input amplifier B and the amplifier D, respectively.

Similar to the peaking amplifier 50 (FIG. 10) and the peaking amplifier60 (FIG. 12), the peaking amplifier 70 includes an untuned, basefeedback amplifier FB_(Base), which is always on. Further, the peakingamplifier 70 includes a tunable feedback amplifier FB_(Tune). Thetunable feedback amplifier FB_(Tune) may be tuned using any of thetuning techniques detailed above (e.g., through adjustment of V_(Tune1)and/or variation in the number of feedback amplifier cells). The inputsof the base feedback amplifier FB_(Base) and the tunable feedbackamplifier FB_(Tune) are connected to the output node V_(Out) of thepeaking amplifier 70. The output of the base feedback amplifierFB_(Base) is connected to the feedback node 12. The output of thetunable feedback amplifier FB_(Tune) is coupled to the output of theinput amplifier B at feedback node 22 and is capacitively coupled to theoutput of the amplifier A by the coupling capacitor C₁.

Unlike the previously disclosed frequency tunable peaking amplifiers,the peaking amplifier 70 also includes a tunable feedback amplifierFB_(Pos), which is included in a positive feedback loop extending fromV_(Out) to the output of the tunable feedback amplifier FB_(Tune) atfeedback node 22. The tunable feedback amplifier FB_(Pos) may be tunedusing any of the tuning techniques detailed above (e.g., throughadjustment of V_(Tune2) and/or variation in the number of feedbackamplifier cells).

The tunable feedback amplifier FB_(Tune) in the peaking amplifier 70 isAC coupled to the feedback node 12 by the coupling capacitor C₁. Thisconfiguration reduces the variation in DC gain attenuation for thepeaking amplifier 70. This configuration also eliminates common modeshift at the output caused by the g_(m) tuning. In order to maintain amoderate level of DC gain attenuation, the untuned, base feedbackamplifier FB_(Base) is connected to the output of the input amplifier A(at the feedback node 12) in a negative feedback configuration. Byleaving the feedback coupling capacitor C_(FB) and the load impedanceZ_(FB) in the peaking amplifier 70, the load degradation through thecapacitor C_(FB) counteracts any gain increase at the output of theinput amplifier B. This requires a relatively large capacitor C_(FB).

FIG. 15 depicts a chart of the frequency response of the peakingamplifier 70 of FIG. 14 according to embodiments (without the tunablefeedback amplifier FB_(Pos)). Advantageously, comparing FIG. 15 withFIGS. 11 and 13, it can be seen that similar to the peaking amplifiers50 and 60, there is a reduced variation in DC gain attenuation for thepeaking amplifier 70. Further, rather than having a gain rolloff athigher frequencies as occurs with the peaking amplifier 50, or a gainincrease at higher frequencies as occurs with the peaking amplifier 60,the gain remains substantially constant over the entire tuning range ofthe peaking amplifier 70.

It can be seen that the tuning range of the peaking amplifier 70(without the tunable feedback amplifier FB_(Pos)) is slightly reducedcompared to some of the above-described frequency tunable peakingamplifiers. This is due to the inclusion of the base feedback amplifierFB_(Base) in the peaking amplifier 70, which limits the bottom end ofthe tuning range. To this extent, as shown in FIG. 14, a tunablefeedback amplifier FB_(Pos) may be connected in a positive feedbackconfiguration in the peaking amplifier 70 and tuned as necessary tocounteract the negative feedback of the base feedback amplifierFB_(Base) at higher frequencies. As described above, the tunablefeedback amplifier FB_(Pos) is included in a positive feedback loopextending from V_(Out) to the output of the tunable feedback amplifierFB_(Tune). The gain of the tunable feedback amplifier FB_(Pos) may betuned, for example, by selectively applying V_(Tune2) to the tunablefeedback amplifier FB_(Pos).

FIG. 16 depicts a chart of the frequency response of the peakingamplifier 70 of FIG. 14 according to embodiments (including the tunablefeedback amplifier FB_(Pos)). Advantageously, comparing FIG. 16 withFIG. 15, it can be seen that not only does the gain remain relativelyconstant over the entire tuning range of the peaking amplifier 70, butalso the tuning range has increased. In this configuration, the peakingamplifier 70 has a tunable center frequency of about 14.5 GHz to 26.4GHz, while maintaining a peak gain that varies a small amount from about13.8 dB to about 13.3 dB to about 13.5 dB across the tuning range. TheDC attenuation remains a constant 9 dB, giving around 24 dB of peaking.The response over this tuning range has an effective 1 dB bandwidth offrom about 11 to 28.4 GHz.

Numerous embodiments of peaking amplifiers 10, 20, 30, 40, 50, 60, and70 are disclosed herein, each having particular operationalcharacteristics. Advantageously, one shared characteristic is amulti-GHz tuning range rather than a single frequency as provided byknown peaking amplifiers. Additional advantages of each embodiment aredescribed in detail above.

Further aspects of the present disclosure provide tunable peakingamplifiers which can be utilized in integrated circuit chips withvarious analog and digital integrated circuitries. For example,integrated circuit dies can be fabricated having peaking amplifiers andother semiconductor devices such as field-effect transistors, bipolartransistors, metal-oxide-semiconductor transistors, diodes, resistors,capacitors, inductors, etc., forming analog and/or digital circuits. Thepeaking amplifiers can be formed upon or within a semiconductorsubstrate. An integrated circuit in accordance with the presentinvention can be employed in applications, hardware, and/or electronicsystems. Suitable hardware and systems for implementing the inventionmay include, but are not limited to, personal computers, communicationnetworks, electronic commerce systems, portable communications devices(e.g., cell phones), solid-state media storage devices, functionalcircuitry, etc. Systems and hardware incorporating such integratedcircuits are considered part of this invention. Given the teachingsprovided above, one of ordinary skill in the art will be able tocontemplate other implementations and applications of the techniques ofdescribed herein.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” “approximately” and “substantially,” are notto be limited to the precise value specified. In at least someinstances, the approximating language may correspond to the precision ofan instrument for measuring the value. Here and throughout thespecification and claims, range limitations may be combined and/orinterchanged, such ranges are identified and include all the sub-rangescontained therein unless context or language indicates otherwise.“Approximately” as applied to a particular value of a range applies toboth values, and unless otherwise dependent on the precision of theinstrument measuring the value, may indicate, for example +/−10% of thestated value(s).

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below”,“lower”, “above”, “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

The foregoing description of various aspects of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to anindividual in the art are included within the scope of the disclosure asdefined by the accompanying claims.

The descriptions of the various embodiments of the present disclosurehave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A peaking amplifier circuit, comprising: a firstinput amplifier having an input connected to an input node and an outputconnected to a first feedback node; a second input amplifier having aninput connected to the input node and an output connected to a secondfeedback node; a capacitor connected between the first feedback node andthe second feedback node; an amplifier having an input connected to thefirst feedback node and an output connected to an output node; a tunablefeedback amplifier in a negative feedback loop with an input connectedto the output node and an output connected to the second feedback node;and a tuning circuit for varying a transconductance of the tunablefeedback amplifier to adjust an operational frequency of the peakingamplifier circuit.
 2. The peaking amplifier circuit of claim 1, whereinthe tuning circuit applies a tuning voltage to the tunable feedbackamplifier.
 3. The peaking amplifier circuit of claim 2, wherein thetuning circuit applies the tuning voltage to a gate of a tail transistorin the tunable feedback amplifier.
 4. The peaking amplifier circuit ofclaim 1, further comprising a series arrangement of a capacitor and aload impedance connected from the first feedback node to a sourcevoltage.
 5. The peaking amplifier circuit of claim 1, further comprisinga tunable feedback amplifier in a positive feedback loop with an inputconnected to the output node and an output connected to the secondfeedback node.
 6. The peaking amplifier circuit of claim 1, wherein thetuning circuit applies a tuning voltage to the tunable feedbackamplifier in the positive feedback loop.
 7. The peaking amplifiercircuit of claim 6, wherein the tuning circuit applies the tuningvoltage to a gate of a tail transistor in the tunable feedback amplifierin the positive feedback loop.
 8. The peaking amplifier circuit of claim2, wherein the tunable feedback amplifier in the negative feedback loopincludes a plurality of tunable feedback amplifier cells, and whereinthe tuning circuit selectively activates or deactivates at least one ofthe tunable feedback amplifier cells.
 9. The peaking amplifier circuitof claim 8, wherein the tuning circuit selectively activates ordeactivates at least one of the tunable feedback amplifier cells byapplying the tuning voltage to a gate of a tail transistor in at leastone of the tunable feedback amplifier cells.
 10. The peaking amplifiercircuit of claim 1, further comprising an integrated circuit includingthe peaking amplifier circuit.
 11. A peaking amplifier circuit,comprising: a first input amplifier having an input connected to aninput node and an output connected to a first feedback node; a secondinput amplifier having an input connected to the input node and anoutput connected to a second feedback node; a first capacitor connectedbetween the first feedback node and the second feedback node; anamplifier having an input connected to the first feedback node and anoutput connected to an output node; a second capacitor and a loadimpedance in series connected from the first feedback node to a sourcevoltage; a first tunable feedback amplifier in a negative feedback loopwith an input connected to the output node and an output connected tothe second feedback node; a second tunable feedback amplifier in apositive feedback loop with an input connected to the output node and anoutput connected to the second feedback node; and a tuning circuit forapplying a first tuning voltage to the first tunable feedback amplifierand for applying a second tuning voltage to the second tunable feedbackamplifier.
 12. The peaking amplifier circuit of claim 11, wherein thetuning circuit applies the first tuning voltage to a gate of a tailtransistor in the first tunable feedback amplifier.
 13. The peakingamplifier circuit of claim 11, wherein the tuning circuit applies thesecond tuning voltage to a gate of a tail transistor in the secondtunable feedback amplifier.
 14. The peaking amplifier circuit of claim11, wherein the first tunable feedback amplifier includes a plurality oftunable feedback amplifier cells, and wherein the tuning circuitselectively activates or deactivates at least one of the tunablefeedback amplifier cells.
 15. The peaking amplifier circuit of claim 14,wherein the tuning circuit selectively activates or deactivates at leastone of the tunable feedback amplifier cells by applying the tuningvoltage to a gate of a tail transistor in at least one of the tunablefeedback amplifier cells.
 16. The peaking amplifier circuit of claim 11,further comprising an integrated circuit including the peaking amplifiercircuit.
 17. A peaking amplifier circuit, comprising: a first inputamplifier having an input connected to an input node and an outputconnected to a first feedback node; a second input amplifier having aninput connected to the input node and an output connected to a secondfeedback node; a capacitor connected between the first feedback node andthe second feedback node; an amplifier with an input connected to thefirst feedback node and an output connected to an output node; a firsttunable feedback amplifier with an input connected to the output nodeand an output connected to the second feedback node; a second tunablefeedback amplifier with an input connected to the output node and anoutput connected to the second feedback node; and a tuning circuit forselectively applying a first tuning voltage to the first tunablefeedback amplifier and a second tuning voltage to the second tunablefeedback amplifier.
 18. The peaking amplifier circuit of claim 17,wherein the first tunable feedback amplifier is in a negative feedbackloop.
 19. The peaking amplifier circuit of claim 17, wherein the secondtunable feedback amplifier is in a positive feedback loop.
 20. Thepeaking amplifier circuit of claim 17, further comprising an integratedcircuit including the peaking amplifier circuit.